The present invention is related to downhole, pneumatically operated, impact drilling tools and more particularly to such tools that are self-aligning within the drilled hole.
Pneumatic rock drilling tools have generally been limited to drilling holes having diameters slightly greater than the tool diameter. When drilled holes having diameters close to the diameter of the tool, the tool casing itself maintains the tool in substantial alignment with the hole axis. That is to say, the outside surfaces of the housing itself are spaced closely enough to the hole walls that substantial misalignment can not occur. It becomes difficult, however, to maintain tools in proper orientation relative to the hole axis when the hole diameter is more than 7.5 centimeters greater than the tool diameter. This problem has been overcome to a limited extent by increasing the thickness of the casing wall of the tool housing to substantially that of the drilled bore or hole diameter. This presents several other problems. Firstly, the tool weight is increased significantly and often results in excessive wear and damage to the associated bit. Another problem is that there is not sufficient clearance to enable air and earth material to escape freely upwardly from the operating drill bit. The peripheral housing of an elongated tool spaced closely adjacent to the hole walls is restrictive and does not allow free upward passage of any sizable earth material. Therefore the downward progress of the drill is hampered. A still further difficulty is in the manufacture of the tools themselves. The housing must be manufactured to the specification of the hole size to be drilled.
It therefore becomes desirable to obtain some form of self-aligning impact rock drilling tool that efficiently performs the function of maintaining the tool in proper central alignment with respect to the axis of the hole being bored and that does not add excessive weight to the tool housing nor impede downward progress of the tool by restricting upward passage of earth material and air. It is also desirable to obtain such a tool that may be manufactured with a housing of a standard size and with elements mounted to the standard size housing that are adapted to the purpose of alignment of the tool within a hole of specified diameter.